Some more thoughts. I think you might already be doing something like this, but to add.
Have trees have a much higher chance of growing on grass vs. desert tiles and also have trees have a much higher chance of dying in deserts vs. grass.

Have trees die occasionally. Replace a living tree with a dead tree.
Have dead trees disappear occasionally. Making space for new trees.

So basically a fully living eco system.

One last thought, let the chance of a tree dying be higher in high pollution areas that now pollution areas. This way it will work very well with the game mechanics and also, ones base won't be overrun by trees.

Yeah, I've been looking into changing it over to a white-list approach, so fertile tiles will grow trees, but non-fertile and all the various mods with "man-made" floor tiles will not allow new trees. I like the idea of killing trees in high pollution areas, that should also help with keeping any lag down to a minimum.
Thanks for your input!